The most important parameters of a bipolar ionizer are the amount of positive and negative ions at the ionizer output and the balance between the concentrations of the positive and negative ions.
Usually in order to attain the required balance, a shield is positioned adjacent to the ionizing electrodes. The shield can be either passive or active. In the latter case offset voltage is applied to the shield to adjust the ion balance in the ions outflow.
Typical arrangements are shown in U.S. Pat. Nos. 3,812,559, 3,909,614, 4,096,543, 4,227,894, 4,618,249, 5,381,789, 6,785,114, 6,850,403, and 7,031,134; US 2006/0072279, RU 2294776 and JP 3700003.
A major drawback of the existing state of art is the low efficiency of the generators stemming from the use of shields. Indeed, because of the proximity of the shield vicinity to the ionizing electrodes, 90-95% of the ion current arrives at the shield, while only 5 to 10% of the current escapes the ionizer and reaches the open air. The drawback results in low accuracy of the opposite polarity ions control outside the ionizer. For example, an imbalance of 10% in the output ion current results in an imbalance of as little as 1% in the overall ion current.
In principle this has a strong impact on the use of such ionizers since it is the user who experiences the conditions for imbalance which results from the installation and the changing conditions of operation.
U.S. Pat. No. 8,705,224 in the name of the present inventor discloses a method for generating ions that includes generating AC high voltage using a stationary AC generator, applying the AC high voltage to one or more AC/DC voltage converters via capacitive air coupling between a high voltage terminal of the AC generator and one or more high voltage terminals of the AC/DC voltage converters. The AC/DC voltage converters are rotated, and each AC/DC voltage converter is connected to an air ionizing electrode that rotates with the AC/DC voltage converter it is connected to when that AC/DC voltage converter is rotated, relative to the AC generator, in an air flow. Additional capacitive air coupling is provided between a low voltage terminal of the AC generator and one or more low voltage terminals of the AC/DC voltage converters.
U.S. Pat. No. 8,611,865 in the name of the present inventor discloses a method and a device for automatic positive and negative ion balance control in a bipolar ion generator. A bias voltage from a bias voltage source is applied to a bias electrode from a power supply that includes an AC voltage generator and a voltage multiplying circuit of at least one cascade. The bias current flowing through the bias electrode may be controlled by charging a capacitor in the voltage multiplying circuit so as stabilize the bias current.